In olefin epoxidation, a feed containing an olefin and an oxygen source is contacted with a catalyst under epoxidation conditions. The olefin is reacted with oxygen to form an olefin oxide. A product mix results that contains olefin oxide and typically unreacted feed and combustion products.
The catalyst comprises silver, usually with one or more additional elements deposited therewith, on a carrier, typically an alpha-alumina carrier. The olefin oxide may be reacted with water to form a 1,2-diol, with an alcohol to form a 1,2-diol ether, or with an amine to form an alkanolamine. Thus, 1,2-diols, 1,2-diol ethers, and alkanolamines may be produced in a multi-step process initially comprising olefin epoxidation and then the conversion of the formed olefin oxide with water, an alcohol, or an amine.
The performance of the silver containing catalyst may be assessed on the basis of selectivity, activity, and stability of operation in the olefin epoxidation. The selectivity is the molar fraction of the converted olefin yielding the desired olefin oxide. As the catalyst ages, the fraction of the olefin reacted normally decreases with time. To maintain a desired constant level of olefin oxide production, the temperature of the reaction is increased. However, increasing the temperature causes the selectivity of the reaction to the desired olefin oxide to decrease. In addition, the equipment used in the reactor typically can tolerate temperatures only up to a certain level. Thus, it may become necessary to terminate the reaction when the reaction temperature reaches a temperature inappropriate for the reactor. Thus, the longer the selectivity can be maintained at a high level and the epoxidation can be performed at an acceptably low reaction temperature while maintaining an acceptable level of olefin oxide production, the longer the catalyst charge can be kept in the reactor and the more product is obtained. Stability refers to how the selectivity and/or activity of the process changes during the time a charge of catalyst is being used, i.e., as more olefin oxide is produced.
At any given time, the selectivity of a process is also dependent on the level of olefin oxide production. As the level of olefin oxide production, which is conveniently measured by the olefin oxide partial pressure in the product mix or mole percentage of olefin oxide in the product mix (i.e., at the reactor outlet), increases, the selectivity of the process decreases. Because of this, typical commercial olefin epoxidation processes are run at relatively low olefin oxide partial pressures, at the sacrifice of potential plant productivity.
Modern silver-based catalysts may comprise, in addition to silver, one or more high-selectivity dopants, such as components comprising rhenium, tungsten, chromium, or molybdenum. High-selectivity catalysts are disclosed, for example, in U.S. Pat. No. 4,761,394 and U.S. Pat. No. 4,766,105. U.S. Pat. No. 4,766,105 and U.S. Pat. No. 4,761,394 disclose that rhenium may be employed as a further component in the silver containing catalyst with the effect that the initial, peak selectivity of the olefin epoxidation is increased.
Depending upon the catalyst used and the parameters of the olefin epoxidation process, the time required to reach the initial, peak selectivity, that is the highest selectivity reached in the initial stage of the process, may vary. For example, the initial, peak selectivity of a process may be achieved after only 1 or 2 days of operation or may be achieved after as much as, for example, 1 month of operation. Working examples given in these US patents show a trend towards a higher selectivity at higher rhenium levels up to about 3 mmole rhenium/kg catalyst, on a carrier having a surface area of 0.42 m2/g. EP-A-352850 also teaches that the then newly developed catalysts, comprising silver supported on alumina carrier, promoted with alkali metal and rhenium components have a very high selectivity.
Not withstanding the improvements already achieved, there is a desire to further improve the performance of olefin epoxidation catalysts, in particular, to increase the level of olefin oxide production without sacrificing the selectivities achievable with the use of the catalysts.